fmt: document GoStringer and explain application of formats to compound objects %q quotes each element of a string slice; this was never explained in the docs. Fixes issue 7015.
fmt: document GoStringer and explain application of formats to compound objects
%q quotes each element of a string slice; this was never explained in the docs.
Fixes issue 7015.

LGTM=josharian
R=golang-codereviews, josharian
CC=golang-codereviews
https://codereview.appspot.com/77140044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang